Assuming we're not talking about those 'cup style 7MM (or what ever they're down to)stems, they last a long time. I just don't know on the very small stem valves. They may last as well. I'm sure heavy nitrous or other severe conditions would shorten their life.
We did a set of ex cup chevy heads way back in the 23° days. Used the same valves, and they saw 8 years in the super classes before I lost touch with them. No valve failures.
So, the material itself seems to be good for long life.